Hi akshu, > dear siesta users > i need to calculate the bulk modulus in order to test the pseudopotential > for technetium. Tc is found in hexagonal structure, so along with varying > the lattice constant, do i need to change 'c' as well?
The closest thing to bulk modulus in hcp is obtained by varying all the cell dimensions simultaneously and homogeneously and monitoring the pressure while doing that. Then P = B \delta V / V. > is it enough to use md.variable cell? how will i know that cell geometry > remains hexagonal? Of course you shouldn't use variable cell: for each \delta V above the cell is kept fixed, otherwise the pressure will relax to zero. > should i fix the atomic positions? No. The hcp 2-atoms cell possesses inner degrees of freedom, most likely the symmetry will be broken. > Is it all right if i test the pseudo for fcc structure instead and compare > the results with available theoretical results. > Who knows. Any test is just a test, it should be helpful anyway. Regards, Roberto
